DE102019135330A1 - Method for operating a reciprocating piston engine - Google Patents

Abstract

Method for operating a reciprocating piston machine (1) which has a crankshaft (3), at least one cylinder (4), at least one in the cylinder (4) between a bottom dead center (BDC) and a top dead center (TDC) in a stroke direction (e.g. ) has reciprocating piston (5) and at least one connecting rod (6) by means of which the piston (5) is connected to the crankshaft (3), with a fuel-air in a combustion chamber (8) of the reciprocating piston engine (1) Mixture (10) is introduced and compressed by the piston (5) until the fuel-air mixture (10) ignites and burns under constant space conditions in the combustion chamber (8), into which a pilot fuel (11) is introduced and burned, the combustion of which has a low-temperature reaction and a subsequent high-temperature reaction, the combustion of the fuel-air mixture (10) under constant space conditions by the time at which the low-temperature reaction of the pilot fuel (11) produces heat ice formation and / or is controlled by the amount of heat released by the low-temperature reaction of the pilot fuel (11).

Description

The invention relates to a method for operating a reciprocating piston machine which has a crankshaft, at least one cylinder, at least one piston that can be moved back and forth in a stroke direction in the cylinder between a lower dead center and an upper dead center, and at least one connecting rod by means of which the piston is connected the crankshaft is connected, a fuel-air mixture being introduced into a combustion chamber of the reciprocating piston engine and compressed by the piston until the fuel-air mixture ignites and burns under constant space conditions in the combustion chamber, into which a pilot fuel is introduced and burned, the combustion of which has a low-temperature reaction and a subsequent high-temperature reaction.

The specific heat capacity of a gas is understood to mean Cv as the isochoric specific heat capacity in the sense of a ratio of the supplied heat and the temperature increase caused at constant volume during the process, and Cp as the isobaric specific heat capacity in the sense of a ratio of the heat supplied and the temperature increase caused at constant pressure during the Litigation.

R = universelle GaskonstanteThe general relationship is simplified for ideal gases as follows: $$\text{Cp}=\text{Cv}+\text{R;}$$

R = universal gas constant

R = Gaskonstante Abgas (R=300 J/kgK)
Vc = Brennraum-Volumen $$\text{p\_isobar}=\left(\text{Q\_Brennstoff*R}\right)/\left(\text{Cp*Vc}\right)$$

p_isobar = Verbrennungsdruck Gleichdruckprozess (Cp)
Q_Brennstoff = Wärmemenge pro Arbeitshub und Zylinder
Beispiel: für 1 mg Diesel ist Q_Brennstoff = 42,3 J $$\text{p\_isochor}=\left(\text{Q\_Brennstoff*R}\right)/\left(\text{Cv*Vc}\right)$$

p_isochor = Verbrennungsdruck Gleichraumprozess (Cv) The significant heat engines, gas turbines and internal combustion engines, usually work in the constant pressure process, i.e. with the isobaric heat capacity Cp. Internal combustion engines are now also working in the Seiliger process, which is a superposition of the constant pressure process and the constant space process. The real thermodynamic constant-space process usually generates combustion pressures that are more than 30% higher, which can be calculated as follows: $$\text{p\_total}=\text{p\_compression}+\text{p\_combustion pressure}$$

R = gas constant exhaust gas (R = 300 J / kgK)
Vc = combustion chamber volume $$\text{p\_isobar}=\left(\text{Q\_Fuel * R}\right)/\left(\text{Cp * Vc}\right)$$

p_isobar = combustion pressure equal pressure process (Cp)
Q_Fuel = amount of heat per working stroke and cylinder
Example: for 1 mg of diesel, Q_fuel = 42.3 J. $$\text{p\_isochor}=\left(\text{Q\_Fuel * R}\right)/\left(\text{Cv * Vc}\right)$$

p_isochor = combustion pressure constant space process (Cv)

In internal combustion engines, the constant space efficiency is significantly higher than the constant pressure efficiency.

The effective constant pressure efficiency is between 30% and 35% for naturally aspirated engines and between 40% and 45% for supercharged engines. The effective constant space efficiency reaches up to 45% (0% EGR) and supercharged engines 53% (0% EGR) and more (EGR = exhaust gas recirculation) with naturally aspirated engines.

In order to set higher effective efficiencies in naturally aspirated engines (for example> 36%), the degree of uniformity of the combustion process must be increased. However, an increase in the degree of constant space leads to a temporary instability of the combustion, since part of the charge, i.e. the fuel-air mixture, burns in the combustion chamber in the constant pressure process, whereas with another part of the charge, constant space combustion takes place at a different location in the combustion chamber. This leads to large pressure jumps in the combustion chamber, which is noticeable as strong knocking.

Only an almost complete constant-space combustion stabilizes the process again, whereby the knocking, which was caused by the constant pressure and constant-space processes running in parallel in the combustion chamber, is now due to the high pressure increase rates of more than 30 bar / degree crankshaft angle ( KW ) is replaced.

A stable constant space process exists when the effective efficiency of naturally aspirated engines in the full load range reaches over 42% and with supercharged combustion engines over 52%.

The implementation and control of the constant space process in the internal combustion engine proves to be very difficult, especially in the full load range. In order to produce a complete constant-space combustion, the combustion must take place at Vc = const. take place and the homogeneous fuel-air mixture in the entire combustion chamber or volume Vc of the combustion chamber are ignited simultaneously.

Since the combustion in gasoline and diesel engines is characterized by the course of a flame front, there is no constant-space combustion, but rather a mixed form (such as the Seiliger process).

The best-known constant-space combustion process is the HCCI process. With homogeneous auto-ignition, a homogeneous, lean fuel-air mixture is introduced into the combustion chamber, which ignites simultaneously in the entire combustion chamber during the compression stroke. The energy turnover is so fast that the combustion takes place at a constant volume (Vc) and so the following applies: $$\text{p}=\left(\text{Q * R}\right)/\left(\text{Cv * Vc}\right)$$

With the same amount of fuel (Q), the combustion pressure is more than 30% higher than with flame front combustion (constant pressure process).

The disadvantage of the HCCI process, in addition to the high pressure gradients, is the inadequate control of the process.

The combustion in the HCCI process takes place in a two-stage ignition, first in a low-temperature reaction ("cool flame") and after a short pause, the high-temperature reaction ("hot flame") takes place, i.e. the actual combustion.

There is thus a desire to initiate real thermodynamic constant-space combustion of the previously introduced fuel-air mixture with the specific heat capacity Cv in the piston position that is optimal for efficiency.

Proceeding from this, the invention is based in particular on the object of being able to better control the constant-space combustion of the fuel-air mixture in a method of the type mentioned at the beginning.

This object is achieved according to the invention by a method according to claim 1. Preferred developments of the invention are given in the subclaims and in the following description.

According to the invention is or is a method for operating a reciprocating piston engine which has a crankshaft, one or at least one cylinder, one or at least one piston that can be moved back and forth in a stroke direction in the cylinder between a bottom dead center and a top dead center and a piston or at least one has a connecting rod, by means of which the piston is connected to the crankshaft, wherein a fuel-air mixture is introduced into a combustion chamber of the reciprocating piston engine, preferably injected, and is compressed by the piston until the fuel-air mixture, preferably itself, ignited and burned under constant space conditions in the combustion chamber, into which a pilot fuel is introduced and burned, the combustion of which has a low-temperature reaction and a subsequent high-temperature reaction, in particular further developed in that the combustion of the fuel-air mixture under constant-space conditions through the time of the low-temperature reaction of the pilot fuel is the release of heat and / or is controlled by the amount of heat released by the low-temperature reaction of the pilot fuel.

The constant-space combustion of the fuel-air mixture can be controlled by the point in time and / or the amount of heat release of the low-temperature reaction of the pilot fuel. In particular, very rapid and almost simultaneous combustion of the fuel-air mixture introduced into the combustion chamber can be achieved. This also makes it possible, for example, to keep the amount of nitrogen oxides produced by the combustion low.

The pilot fuel is preferably injected into the combustion chamber. For example, the pilot fuel is introduced or injected into the combustion chamber after the introduction or injection of the fuel-air mixture and / or before the ignition of the fuel-air mixture and / or before the top dead center is reached. The pilot fuel preferably has greater ignitability than the fuel-air mixture, in particular such that the pilot fuel ignites, preferably by itself, and combustion of the pilot fuel begins, in particular, before the combustion of the fuel-air mixture begins.

The amount of the pilot fuel introduced into the combustion chamber is preferably smaller or very much smaller than the amount of the fuel-air mixture introduced into the combustion chamber. For example, the pilot fuel is introduced into the combustion chamber as such or in the form of a pilot fuel-air mixture. The combustion of the pilot fuel takes place in particular in the combustion chamber.

The piston preferably moves to and fro in the cylinder between the bottom dead center and the top dead center in the stroke direction. The crankshaft is or is preferably rotatable or rotated about a crankshaft axis of rotation. The crankshaft is or is advantageously drivable or driven by means of the piston, preferably in such a way that the crankshaft is or is rotated about the crankshaft axis of rotation. The rotational angle position of the crankshaft is or is defined and / or characterized in particular by the or a crankshaft angle. The or a crankshaft angle is preferably abbreviated with the expression KW. The top dead center is abbreviated in particular with the term OT. Furthermore, the bottom dead center is preferably abbreviated with the expression UT.

The combustion chamber is advantageously provided, in particular in the stroke direction, between the piston and the cylinder or a head of the cylinder. The head of the cylinder is also referred to as the cylinder head, for example. In particular, the combustion chamber is provided within the cylinder. The combustion chamber is also referred to as the working chamber, for example. The connecting rod is also known as a push rod, for example. Instead of the expression “compress”, the expression “compress” can also be used and vice versa. The introduction and / or injection of the pilot fuel into the combustion chamber is referred to in particular as pilot injection.

Equal space conditions in the combustion chamber preferably exist in a range between 10 degrees crankshaft angle before top dead center to 10 degrees crankshaft angle after top dead center. This is due in particular to the fact that the volume of the combustion chamber does not change or does not change significantly in this area. The ignition and / or the combustion of the fuel-air mixture preferably takes place in a range between top dead center and 10 degrees crankshaft angle after top dead center. The ignition and / or combustion of the fuel-air mixture preferably takes place in a range between 3 degrees crankshaft angle after top dead center and 10 degrees crankshaft angle after top dead center. It can thus be avoided in particular that the combustion works against the movement of the piston. The introduction or injection of the pilot fuel advantageously takes place before the top dead center is reached and / or before the piston reaches the top dead center.

The ignition of the fuel-air mixture is preferably compression-induced self-ignition. The ignition of the pilot fuel is preferably compression-induced self-ignition.

The ignition and / or the combustion of the pilot fuel leads in particular to an increase in temperature of the fuel-air mixture in the combustion chamber and / or to an increase in temperature in the combustion chamber. The combustion of the pilot fuel preferably ends after the fuel-air mixture has ignited.

In the high-temperature reaction of the pilot fuel, in particular, more heat is released than in the low-temperature reaction of the pilot fuel. The low-temperature reaction of the pilot fuel preferably begins before the fuel-air mixture is ignited. The low-temperature reaction of the pilot fuel preferably increases the temperature of the fuel-air mixture in the combustion chamber and / or the temperature in the combustion chamber. The high-temperature reaction of the pilot fuel advantageously takes place at a higher temperature than the low-temperature reaction of the pilot fuel. In particular, there is a, preferably short, pause between the low-temperature reaction of the pilot fuel and the high-temperature reaction of the pilot fuel. The low-temperature reaction of the pilot fuel is preferably brought about and / or initiated and / or induced by the compression of the fuel-air mixture, in particular together with the pilot fuel.

According to one embodiment, the combustion of the fuel-air mixture has a low-temperature reaction and a high-temperature reaction that follows this. In particular, more heat is released in the high-temperature reaction of the fuel-air mixture than in the low-temperature reaction of the fuel-air mixture. The low-temperature reaction of the fuel-air mixture preferably begins during or after the low-temperature reaction of the pilot fuel. The low-temperature reaction of the fuel-air mixture is advantageously initiated by the low-temperature reaction of the pilot fuel. The high-temperature reaction of the fuel-air mixture advantageously takes place at a higher temperature than the low-temperature reaction of the fuel-air mixture. In particular, there is a, preferably short, pause between the low-temperature reaction of the fuel-air mixture and the high-temperature reaction of the fuel-air mixture. The high-temperature reaction of the fuel-air mixture forms, in particular, a focus of the combustion of the fuel-air mixture and / or is, for example, also referred to as the combustion focus. The combustion of the fuel-air mixture and / or the high-temperature reactions of the fuel-air mixture preferably drives the piston, which preferably, in turn, drives the crankshaft.

The high-temperature reaction of the pilot fuel and the high-temperature reaction of the fuel-air mixture preferably take place at the same time, in particular at least temporarily and / or temporarily and / or at least partially and / or partially and / or essentially at the same time. The low-temperature reaction of the pilot fuel and the low-temperature reaction of the fuel-air mixture are preferably completed before the high-temperature reaction of the pilot fuel and the high-temperature reaction of the fuel-air mixture.

The combustion of the fuel-air mixture under constant space conditions is controlled in particular by the point in time at which the heat is released by the low-temperature reaction of the pilot fuel and / or by the amount of heat released by the low-temperature reaction of the pilot fuel. Complementary or alternatively, the combustion of the fuel-air mixture under constant room conditions is controlled, for example, by the point in time of the heat release caused by the low-temperature reaction of the fuel-air mixture and / or by the amount of heat released by the low-temperature reaction of the fuel-air mixture.

For example, the combustion of the fuel-air mixture, in particular additionally, is controlled by the point in time of the introduction and / or the injection of the pilot fuel and / or by the ignitability of the pilot fuel. The or a compression ratio and / or the or a temperature or intake temperature of the air and / or the fuel-air mixture is or is preferably set such that a compression temperature of at least 720 K is between 12 degrees crankshaft angle before top dead center and 0 degrees crankshaft angle is reached before top dead center. The aforementioned air is advantageously the air of the fuel-air mixture. The air of the fuel-air mixture is preferably sucked in, for example from the environment, in particular before the introduction and / or injection of the fuel-air mixture.

According to one embodiment, the low-temperature reaction of the pilot fuel increases the pressure in the combustion chamber and / or increases the temperature in the combustion chamber from at least 720 K to 800 K to 1000 K, so that in particular the or a low-temperature reaction of the fuel-air mixture is initiated.

For example, in the low-temperature reaction of the pilot fuel from the particles and / or molecules of the pilot fuel, especially reactive radicals are formed, e.g. by splitting (cracking) the molecules of the pilot fuel. In particular, during the high-temperature reaction of the pilot fuel, the aforementioned radicals are chemically converted, for example with oxygen, preferably from the fuel-air mixture.

For example, in the low-temperature reaction of the fuel-air mixture from the particles and / or molecules of the fuel in the fuel-air mixture, particularly reactive radicals are formed, e.g. by splitting (cracking) the molecules of the fuel. In particular, in the high-temperature reaction of the fuel-air mixture, the chemical conversion of the aforementioned radicals takes place, for example with oxygen, preferably from the fuel-air mixture.

According to one development, the cetane number of the pilot fuel is greater than the cetane number of the fuel-air mixture. For example, the cetane number of the pilot fuel is in the range from 30 to 100. The cetane number is, in particular, a measure of the ignitability of a substance.

The fuel of the fuel-air mixture is or comprises, in particular, a gas. For example, fuel of the fuel-air mixture is or comprises methane and / or natural gas and / or propane. The pilot fuel is preferably or comprises an oil. For example, the pilot fuel is or includes, for example, diesel fuel and / or vegetable oil and / or heating oil.

The most efficient piston position for constant space combustion is, depending on the size of the engine and the push rod ratio, in particular 3 degrees to 10 degrees of crankshaft angle ( KW ) after top dead center ( OT ). In particular, a further shift of the combustion center of gravity after top dead center with constant space combustion has not proven to be expedient, since the reaction kinetics are four to six times faster than gasoline and diesel processes and the constant space process would be destroyed and the combustion would be dragged further into the expansion stroke.

The control of the center of combustion of the previously introduced fuel-air mixture in the piston position for optimum efficiency takes place in particular through the low-temperature reaction (“cool flame”) of the pilot injection. For example, fuels with a cetane number of 30 to 100 are suitable as pilot fuels. Diesel fuel, for example, releases 12% to 14% of its energy in the low-temperature reaction ("cool flame"). The higher the cetane number, the higher the heat release in the low-temperature reaction.

The “cool flame” of the pilot injection has the task of increasing the pressure and the temperature in the combustion chamber from at least 720 K to 850 K to 1000 K, so that a low-temperature reaction of the previously introduced fuel-air mixture is initiated in the combustion chamber. The low-temperature reaction of the pilot injection prevents, in particular, a premature ignition of the fuel-air mixture previously introduced, since the temperature for the high-temperature reaction is not yet reached in the low-temperature reaction.

The compression ratio and the intake temperature of the air and / or the fuel-air mixture is or are preferably set such that a compression temperature of at least 720 K is between 12 degrees KW in front OT and 0 degrees KW in front OT or between 12 degrees KW in front OT and 6 degrees KW in front OT is achieved. Due to the ignition delay of the pilot injection and the fuel-air mixture, the high-temperature reaction (“hot flame”) starts especially after top dead center, i.e. preferably in the Efficiency-optimized piston position. If the piston is in top dead center, this piston position can also be through 0 degrees, for example KW in front OT and / or 0 degrees KW to OT are designated.

The relevant control parameters of the main combustion (“hot flame”) of the fuel-air mixture in the piston position with optimum efficiency are in particular the time and / or amount of the low-temperature heat release from the pilot injection and / or the fuel-air mixture. The temperature or intake temperature, in particular of the air or the fuel-air mixture, and / or the compression ratio, for example, are also added as secondary control parameters. In the case of very lean fuel-air mixtures, for example, an increase in the intake temperature should be aimed for.

The low-temperature reactions ("cool flame") of the pilot injection and the fuel-air mixture preferably take place in quick succession and / or partially in parallel, so that the main combustion ("hot flame") of the pilot injection and the fuel-air mixture are at the same time with optimum efficiency Piston position take place. The low-temperature reaction (“cool flame”) of the fuel-air mixture is particularly a prerequisite for simultaneous ignition and / or combustion in the entire combustion chamber.

One difference to the known pilot injection method is in particular that the low-temperature reaction of the fuel-air mixture did not take place, since the pilot injection in the pilot injection method is shortly before OT and at, preferably significantly, higher compression temperatures, so that an immediate ignition of the pilot injection and thus an immediate ignition of the fuel-air mixture takes place. Preferably, in particular furthermore, the injection quantity of the pilot fuel is so small that no, in particular noteworthy or substantial, temperature increases in the combustion chamber occur during the low-temperature reaction of the pilot injection. The result is that the combustion process corresponds in particular to a turbulent flame front combustion, which accordingly lasts four to six times longer than the constant-space combustion.

According to one embodiment, the method controls and / or initiates, preferably real, thermodynamic constant-space combustion in the reciprocating piston engine. In particular, the point in time and / or the amount of low-temperature heat release (“cool flame”) of the pilot injection and / or of the fuel-air mixture are primary control parameters.

According to a further development, the or a compression ratio and / or the temperature or intake temperature of the air and / or the fuel-air mixture is or will be set so that a compression temperature of at least 720 K between 12 degrees of crankshaft angle before OT up to 0 degrees crankshaft angle OT is achieved.

According to one embodiment, the low-temperature reaction (“cool flame”) of the pilot injection increases the pressure and / or the temperature in the combustion chamber from at least 720 K to 850 K to 1000 K, so that in particular the or a low-temperature reaction (“cool flame”) of the previously introduced fuel -Air mixture is initiated in the combustion chamber.

According to one development, the low-temperature reaction (“cool flame”) of the pilot injection and the previously introduced fuel-air mixture is or must be completed before the high-temperature reaction (“hot flame”), in particular of the pilot fuel and / or the fuel-air mixture .

• 1 eine schematische Ansicht einer Hubkolbenmaschine.

The invention is described below on the basis of a preferred embodiment with reference to the drawing. In the drawing show:

• 1 a schematic view of a reciprocating engine.

Out 1 is a reciprocating engine 1 with one around a crankshaft axis 2 rotatable crankshaft 3 , a cylinder 4th , one in the cylinder 4th between a bottom dead center UT and a top dead center OT in one stroke direction z reciprocating piston 5 and at least one connecting rod 6th can be seen by means of which the piston 5 with the crankshaft 3 connected is. The reciprocating engine also includes 1 one inside the cylinder 4th between the piston 5 and a cylinder head 7th intended combustion chamber 8th . The angle of rotation of the crankshaft 3 with respect to a reference position 9 is given by a crankshaft angle KW characterized. Thereby prevail in the combustion chamber 8th in a range between 10 degrees KW in front OT and 10 degrees KW to OT Equal space conditions in which the volume of the combustion chamber 8th remains essentially constant.

In the combustion chamber 8th the reciprocating engine 1 becomes a fuel-air mixture 10 introduced and through the piston 5 condensed. After introducing the fuel-air mixture 10 is in the combustion chamber 8th a pilot fuel 11 with a greater ignitability than the fuel-air mixture 10 introduced so that the pilot fuel 11 ignites due to compression and a combustion of the pilot fuel 11 begins before combustion of the fuel-air mixture 10 begins.

The combustion of the pilot fuel 11 exhibits a low-temperature reaction and a subsequent high-temperature reaction. Furthermore, the combustion of the fuel-air mixture 10 a low-temperature reaction and a subsequent high-temperature reaction. This is the low-temperature reaction of the fuel-air mixture 10 by the low temperature reaction of the pilot fuel 11 initiated. The low-temperature reaction of the fuel-air mixture preferably begins 10 after or during the low temperature reaction of the pilot fuel 11 . The high-temperature reactions of the pilot fuel which follow the respective low-temperature reaction advantageously continuously 11 and the fuel-air mixture 10 at least temporarily at the same time.

The high temperature reactions of the fuel-air mixture 10 occur in a range between 3 degrees KW to OT up to 10 degrees KW to OT and thus under constant space conditions. It also drives the high temperature reaction of the fuel-air mixture 10 the piston 5 which in turn is the crankshaft 3 drives.

List of reference symbols

1

Reciprocating engine

2

Crankshaft rotation axis

3

crankshaft

4th

cylinder

5

piston

6th

Connecting rod

7th

Cylinder head

8th

Combustion chamber

9

Reference position

10

Fuel-air mixture

11

Pilot fuel

UT

bottom dead center

OT

Top Dead Center

KW

Crankshaft angle

z

Stroke direction

Claims (8)

Method for operating a reciprocating piston machine (1) which has a crankshaft (3), at least one cylinder (4), at least one in the cylinder (4) between a bottom dead center (BDC) and a top dead center (TDC) in a stroke direction (e.g. ) has reciprocating piston (5) and at least one connecting rod (6) by means of which the piston (5) is connected to the crankshaft (3), with a fuel-air in a combustion chamber (8) of the reciprocating piston engine (1) Mixture (10) is introduced and compressed by the piston (5) until the fuel-air mixture (10) ignites and burns under constant space conditions in the combustion chamber (8), into which a pilot fuel (11) is introduced and burned, the combustion of which has a low-temperature reaction and a subsequent high-temperature reaction, characterized in that the combustion of the fuel-air mixture (10) under constant space conditions by the time of the low-temperature reaction of the pilot fuel s (11) and / or is controlled by the amount of heat released by the low-temperature reaction of the pilot fuel (11). Procedure according to Claim 1 , characterized in that the low-temperature reaction of the pilot fuel (11) begins before the fuel-air mixture (10) is ignited and the temperature in the combustion chamber (8) increases. Procedure according to Claim 1 or 2 , characterized in that the combustion of the fuel-air mixture (10) has a low-temperature reaction and a subsequent high-temperature reaction. Procedure according to Claim 3 , characterized in that the low-temperature reaction of the fuel-air mixture (10) begins during or after the low-temperature reaction of the pilot fuel (11). Procedure according to Claim 3 or 4th , characterized in that the low-temperature reaction of the fuel-air mixture (10) is initiated by the low-temperature reaction of the pilot fuel (11). Method according to one of the Claims 3 to 5 , characterized in that the high temperature reaction of the pilot fuel (11) and the high temperature reaction of the fuel-air mixture (10) run at least temporarily simultaneously. Method according to one of the Claims 3 to 6th , characterized in that the low-temperature reaction of the pilot fuel (11) and the low-temperature reaction of the fuel-air mixture (10) are completed before the high-temperature reaction of the pilot fuel (11) and the high-temperature reaction of the fuel-air mixture (10). Method according to one of the preceding claims, characterized in that the cetane number of the pilot fuel (11) is greater than the cetane number of the fuel-air mixture (10).

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